I think the idea was more useful in the 1800's when gut or silk strings were used. The higher tuning helped the bass project a little better. Now that we have much louder modern strings, tuning up a step is less necessary.

The practice began before the advent of steel strings as a way to make the gut strings brighter which I suppose is desireable for solo playing, and it helps the bass stand out from the rest of the ensemble.

Solo tuning is still widely practiced even since the invention of steel strings. I don't quite understand why. Modern solo steel strings are designed to come to pitch at the same tension as standard/orchestral strings, yet somehow they still sound brighter.